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SubjectRe: [PATCH 6/11] ksm: remove old stable nodes more thoroughly
On Fri, Jan 25, 2013 at 06:01:59PM -0800, Hugh Dickins wrote:
> Switching merge_across_nodes after running KSM is liable to oops on stale
> nodes still left over from the previous stable tree. It's not something
> that people will often want to do, but it would be lame to demand a reboot
> when they're trying to determine which merge_across_nodes setting is best.
>
> How can this happen? We only permit switching merge_across_nodes when
> pages_shared is 0, and usually set run 2 to force that beforehand, which
> ought to unmerge everything: yet oopses still occur when you then run 1.
>

When reviewing patch 1, I missed that the pages_shared check would prevent
most of the problems I was envisioning with leftover entries in the
stable tree. Sorry about that.

> Three causes:
>
> 1. The old stable tree (built according to the inverse merge_across_nodes)
> has not been fully torn down. A stable node lingers until get_ksm_page()
> notices that the page it references no longer references it: but the page
> is not necessarily freed as soon as expected, particularly when swapcache.
>
> Fix this with a pass through the old stable tree, applying get_ksm_page()
> to each of the remaining nodes (most found stale and removed immediately),
> with forced removal of any left over. Unless the page is still mapped:
> I've not seen that case, it shouldn't occur, but better to WARN_ON_ONCE
> and EBUSY than BUG.
>
> 2. __ksm_enter() has a nice little optimization, to insert the new mm
> just behind ksmd's cursor, so there's a full pass for it to stabilize
> (or be removed) before ksmd addresses it. Nice when ksmd is running,
> but not so nice when we're trying to unmerge all mms: we were missing
> those mms forked and inserted behind the unmerge cursor. Easily fixed
> by inserting at the end when KSM_RUN_UNMERGE.
>
> 3. It is possible for a KSM page to be faulted back from swapcache into
> an mm, just after unmerge_and_remove_all_rmap_items() scanned past it.
> Fix this by copying on fault when KSM_RUN_UNMERGE: but that is private
> to ksm.c, so dissolve the distinction between ksm_might_need_to_copy()
> and ksm_does_need_to_copy(), doing it all in the one call into ksm.c.
>
> A long outstanding, unrelated bugfix sneaks in with that third fix:
> ksm_does_need_to_copy() would copy from a !PageUptodate page (implying
> I/O error when read in from swap) to a page which it then marks Uptodate.
> Fix this case by not copying, letting do_swap_page() discover the error.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> include/linux/ksm.h | 18 ++-------
> mm/ksm.c | 83 +++++++++++++++++++++++++++++++++++++++---
> mm/memory.c | 19 ++++-----
> 3 files changed, 92 insertions(+), 28 deletions(-)
>
> --- mmotm.orig/include/linux/ksm.h 2013-01-25 14:27:58.220193250 -0800
> +++ mmotm/include/linux/ksm.h 2013-01-25 14:37:00.764206145 -0800
> @@ -16,9 +16,6 @@
> struct stable_node;
> struct mem_cgroup;
>
> -struct page *ksm_does_need_to_copy(struct page *page,
> - struct vm_area_struct *vma, unsigned long address);
> -
> #ifdef CONFIG_KSM
> int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, int advice, unsigned long *vm_flags);
> @@ -73,15 +70,8 @@ static inline void set_page_stable_node(
> * We'd like to make this conditional on vma->vm_flags & VM_MERGEABLE,
> * but what if the vma was unmerged while the page was swapped out?
> */
> -static inline int ksm_might_need_to_copy(struct page *page,
> - struct vm_area_struct *vma, unsigned long address)
> -{
> - struct anon_vma *anon_vma = page_anon_vma(page);
> -
> - return anon_vma &&
> - (anon_vma->root != vma->anon_vma->root ||
> - page->index != linear_page_index(vma, address));
> -}
> +struct page *ksm_might_need_to_copy(struct page *page,
> + struct vm_area_struct *vma, unsigned long address);
>
> int page_referenced_ksm(struct page *page,
> struct mem_cgroup *memcg, unsigned long *vm_flags);
> @@ -113,10 +103,10 @@ static inline int ksm_madvise(struct vm_
> return 0;
> }
>
> -static inline int ksm_might_need_to_copy(struct page *page,
> +static inline struct page *ksm_might_need_to_copy(struct page *page,
> struct vm_area_struct *vma, unsigned long address)
> {
> - return 0;
> + return page;
> }
>
> static inline int page_referenced_ksm(struct page *page,
> --- mmotm.orig/mm/ksm.c 2013-01-25 14:36:58.856206099 -0800
> +++ mmotm/mm/ksm.c 2013-01-25 14:37:00.768206145 -0800
> @@ -644,6 +644,57 @@ static int unmerge_ksm_pages(struct vm_a
> /*
> * Only called through the sysfs control interface:
> */
> +static int remove_stable_node(struct stable_node *stable_node)
> +{
> + struct page *page;
> + int err;
> +
> + page = get_ksm_page(stable_node, true);
> + if (!page) {
> + /*
> + * get_ksm_page did remove_node_from_stable_tree itself.
> + */
> + return 0;
> + }
> +
> + if (WARN_ON_ONCE(page_mapped(page)))
> + err = -EBUSY;
> + else {
> + /*

It will probably be very obvious to people familiar with ksm.c but even
so maybe remind the reader that the pages must already have been unmerged

* This page must already have been unmerged and should be stale.
* It might be in a pagevec waiting to be freed or it might be
......



> + * This page might be in a pagevec waiting to be freed,
> + * or it might be PageSwapCache (perhaps under writeback),
> + * or it might have been removed from swapcache a moment ago.
> + */
> + set_page_stable_node(page, NULL);
> + remove_node_from_stable_tree(stable_node);
> + err = 0;
> + }
> +
> + unlock_page(page);
> + put_page(page);
> + return err;
> +}
> +
> +static int remove_all_stable_nodes(void)
> +{
> + struct stable_node *stable_node;
> + int nid;
> + int err = 0;
> +
> + for (nid = 0; nid < nr_node_ids; nid++) {
> + while (root_stable_tree[nid].rb_node) {
> + stable_node = rb_entry(root_stable_tree[nid].rb_node,
> + struct stable_node, node);
> + if (remove_stable_node(stable_node)) {
> + err = -EBUSY;
> + break; /* proceed to next nid */
> + }

If remove_stable_node() returns an error then it's quite possible that it'll
go boom when that page is encountered later but it's not guaranteed. It'd
be best effort to continue removing as many of the stable nodes anyway.
We're in trouble either way of course.

Otherwise I didn't spot a problem so as weak as it is due my familiarity
with KSM;

Acked-by: Mel Gorman <mgorman@suse.de>

--
Mel Gorman
SUSE Labs


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